COMPREHENSIVE FLOOD PLAIN MANAGEMENT1

Flooding and the susceptibility to flood damage inherent in all land uses constitute the flood hazard. Resolution of the hazard while properly recognizing flood plain environmental attributes within the context of overall community or area needs is the essence of comprehensive flood plain management. The traditional approach–flood control–has effected modification of only the flooding component of the hazard whether it be coastal or inland. Until recently Federal programs have overlooked the possibilities of modifying the susceptibility component, for which the major responsibility lies with non-Federal interests. Beginning with actions in the TVA area, the latter is now being strongly encouraged through Federal programs and actions notably the Flood Plain Management Services and Survey Programs of the Corps of Engineers, those stemming from Executive Order 11296, and those required for eligibility under the National Flood Insurance Act of 1968. Flood plain management objectives must be stated in planning, e.g., economic efficiency, reduction in threat to life and health, environmental improvement, and regional development, to permit proper evaluation of the optional means and approaches for achieving them.     

FLOODPLAIN ANALYSIS,SIMPLIFIED VS. COMPLEX METHODS: A CASE STUDY1

ABSTRACT: The accurate and reliable determination of floodplains, floodway boundaries, and flood water elevations are integral requirements of Flood Insurance Studies. These studies are intended to be used for determining the flood insurance rates. Therefore, the accuracy of the water surface profiles are important. To ensure the high degree of accuracy, the HUD Flood Insurance Administration has developed standards which must be met in the analysis of water surface profiles. A somewhat less accurate study is required for the preparation of Flood Emergency Plans. As part of the flood insurance studies of eight locations in the State of North Dakota, various flood hazard and floodplain information reports were reviewed. The hydrologic and hydraulic analyses, especially the computation of the 100-year water surface profiles, were completed using both simplified and complex hydraulic computation methods. Significant differences were found (1 to 3 feet) between the profiles computed by the SCS simplified method and those computed by HEC-2 computer program. However, the floodplain boundaries determined by both methods were found to be similar. Approximate methods are recommended for rapid determination of the floodplain, floodway boundaries, and inundation area mapping, while sophisticated computer programs (HEC-2) are recommended to be used for developing areas where the 100-year flood elevation has a significant impact on the cost of land development.     

FLOOD-RUNOFF FORECASTING WITH HEC1F1

HEC1F is a computer program for making short- to medium-term forecasts of uncontrolled flood runoff. The program employs unit hydrographs and hydrologic routing to simulate runoff from a subdivided basin. Estimates of future rainfall can be accommodated. Runoff parameters for gaged headwater subbasins can be estimated (optimized) in real time. Blending of calculated with observed hydrographs can be performed. HEC1F is a component of an on-line software system that includes capability for data acquisition and processing, precipitation analysis, streamflow forecasting, reservoir system analysis, and graphical display of data and simulation results. The conceptual framework for HEC1F is described, and application of the program is illustrated.     

STATE-OF-THE-ART OF ESTIMATING FLOOD DAMAGE IN URBAN AREAS1

ABSTRACT: With implementation of the Flood Insurance Act of 1968 many additional local flood protection projects are being considered. Consulting engineers and local agencies need consistent methods to estimate flood damage in order to perform feasibility studies. Federal agencies have a great deal of data and long experience in making damage estimates but no comprehensive guides are available at the local level. Curves of flood damages to different residential structure types are presented. The relationships in use by the U. S. Federal Insurance Administration are shown to be reasonable and are recommended for use as approximate guides. Additional research is recommended and discussion of the paper is invited in order to make additional data available in the literature.     

FLOOD MANAGEMENT IN THE LOWER INCOMATI RIVER BASIN,MOZAMBIQUE: TWO ALTERNATIVES1

ABSTRACT: The aim of this paper is to compare two views of flood management and thus add to the present thinking of living with floods as opposed to the traditional approach of flood control. The traditional pathway has widely been adopted in developed countries and aims to control floodwaters by means of dams and dikes. The alternative pathway tends towards a policy whereby society lives with the floods by being prepared and having the right damage reduction measures in place. In this paper two pathways are tentatively compared for the Lower Incomati Basin, Mozambique. In the design cultural theory is considered, as is how the design of each path may look according to different management perspectives. The Lower Incomati Basin provides an interesting case study as it is in a relatively undeveloped state. Hence, it is an ideal area for conducting research into the application of alternative flood management strategies. The preliminary results suggest that both pathways are feasible. However, considering recent hydrological extremes such as the 2000 floods, the resilient pathway may ultimately be a more appealing flood management strategy.     

OPTIMIZING FLOOD CONTROL ALLOCATION FOR A MULTIPURPOSE RESERVOIR1

ABSTRACT. In the last decade much research has been devoted to applying the systems analysis approach to water resources problems. A popular research goal has been determination of the “best” method of operating a multipurpose reservoir. The goal of this study was to derive the economically optimum flood control diagram for a multipurpose reservoir by systems analysis. The technique employed to optimize the flood control diagram was programmed so that the optimization process could be applied to other multipurpose reservoirs. Two computer programs developed at the U.S. Army Corps of Engineers' Hydrologic Engineering Center were utilized with modifications to simulate the operation of Folsom Reservoir in central California. Economic analyses were incorporated along with an optimization technique into the reservoir operations program; and the resultant program was capable of routing a sequence of monthly reservoir inflows, computing benefits for various flood control diagrams (as dictated by the optimization procedure), and selecting the economically optimum flood control diagram. The univariate gradient technique was the optimization procedure employed. The two computer programs are on file at the Hydrologic Engineering Center in Davis, California.     

EFFECT OF URBANIZATION ON ALTERNATIVE FLOOD CONTROL STRATEGIES1

ABSTRACT: The effect of urbanization on alternative flood control strategies was investigated for a large developing watershed in Texas. Urban and rural areas were modeled separately using a geographically-referenced data base and the U.S. Corps of Engineers HEC-1 and HEC-2 programs, and results yielded a double-peaked hydrograph. Hydrograph input parameters were modified to predict the effects of a wide range of management alternatives including on-site storage, reservoirs, channelization, and development controls. Results indicated a combination of alternatives is required to protect existing and future developments.     

FLOOD MANAGEMENT: WHO BENEFITS AND WHO PAYS1

ABSTRACT: The distribution of costs and benefits among groups in society of alternative flood management actions is examined in terms of their final incidence or resting place-their influence on real estate values. The alternatives examined include structures, disaster relief, floodproofing, watershed land treatment, floodplain zoning, and flood insurance. The impact of each alternative on the amount of flood disability cost paid by the landowner is examined and variations among alternatives are used to explain the past political success of structures and relief as compared to zoning, floodproofing, and insurance.     

NEAR REAL-TIME MAPPING OF THE 1975 MISSISSIPPI RIVER FLOOD IN LOUISIANA USING LANDSAT IMAGERY1

ABSTRACT: The project described in this report was undertaken by the Louisiana State Planning Office to establish the extent of backwater flooding in Louisiana in April 1975. Band 7 Landsat imagery, enlarged to a scale of 1:250,000 was used to visually identify flooded areas. Inundated areas were delineated on overlays keyed to 1:250,000 U.S. Geological Survey topographic quadrangles. Tabular data identifying acres flooded, according to land use type, were derived by merging the flood map overlays with computerized 1972 land use data. Approximately 1.12 million acres of the state were inundated by flood waters. The total acreage and land use types affected by flooding were determined within 72 hours from the time the flood areas were imaged. Flooded maps were prepared for 26 parishes. Field observations were made by Louisiana Cooperative Extension Service county agents in order to determine the accuracy of parish flood maps and flood acreage figures by land use type. Results indicated that this was a fast, accurate, and relatively inexpensive method of compiling flood data for disaster planning and postflood analysis.     

LAKE LEVEL CONTROL AND MANAGEMENT - A CASE STUDY1

ABSTRACT: The frequent high water levels in Chisago Chain of Lakes, located in east-central Minnesota, have caused extensive flood damages. Recent floods raised the concern of the local property owners and they pressured the Chisago County Board of Managers to initiate a study of alternative lake control levels. A study was carried out to identify potential flood control alternatives, screen out the most promising feasible alternatives, and recommend the most cost-effective flood control measure. Several flood control alternatives were considered - eight of them were analyzed and evaluated in detail. A statistical method was used to estimate the expected annual flood damages under existing and future conditions. The effect of all proposed control measures on the annual flood damage reductions (benefits) were determined. Detailed benefit/cost analyses were carried out to evaluate the economic feasibility of alternatives. The effect of potential flood control measures on the environment was also studied. The economic analysis of the most cost-effective alternative did not strongly support artificial lake level control, therefore the decision-making authorities were even more firm in their position to maintain the present condition and chose the Null Alternative as the most suitable alternative.     

FLOODPLAIN MANAGEMENT NEEDS PECULIAR TO ARID CLIMATES1

ABSTRACT: Many practices followed uniformly nationwide in the federal flood control and floodplain management programs are inappropriate or even counter productive in the arid Utah climate. An analysis of the 130-year Utah flood history, the structural and nonstructural flood programs in the state, and local perceptions obtained by field visits and interviews in 35 Utah communities revealed a number of such inefficiencies. Since flood flows dissipate quickly when they emerge from mountain watersheds onto desert lowlands, risks are concentrated near the apex of alluvial fans, include hazard from mud as well as water flow, and are compounded by canal interception of flood waters. Because of variation in the area flooded from one event to the next, floodplain mapping has tended to show risks too high in mapped areas and too low outside. Traditional channelization carries floods downstream past where they would dissipate naturally. The federal government needs to become more active in developing better flood hazard delineation and structural and nonstructural designs for arid areas. State government can help by providing a forum where communities can exchange experiences, reviewing structural designs prepared by local government, and providing local communities with technical expertise for dealing with federal agencies.     

ANALYZING URBANIZATION IMPACTS ON PENNSYLVANIA FLOOD PEAKS1

ABSTRACT: The impact of man made change on the hydrology of developing watersheds is frequently measured in terms of the ratio: flood peak after development to flood peak before development over a range of return periods. However, the analysis of urbanization effects on flood frequency presents a vexing problem because of a general lack of flood data in urban areas and also because of nonstationarity in the development process. Clearly, the flood peak ratio depends on the impervious fraction and percent of basin sewered and these factors have been taken into account in recent urban flood peak models. In genral, these models are developed either by: (1) split sample analysis of available annual flood data, or (2) by computer simulation using mathematical watershed models capable of representing man made changes. The present paper discusses the results of work in progress to characterize the impact of urbanization on small developing watersheds in Pennsylvania.     

Adapting a holistic approach to flood management in the Hawkesbury–Nepean region: complexities and perceptions of the agencies involved

This paper looks into the complexity of managing flood risks in the Hawkesbury–Nepean catchment, Australia. Several aspects are explored: (1) the complexities created by the way different agencies are involved in assessing flood risks; (2) different perceptions on acceptable flood risk level; (3) community engagement in defining acceptable flood risk level; (4) views on a holistic flood risk management plan; and (5) challenges of a centralised information system. This study concludes that the complexity of managing a large catchment is exacerbated by the difference in the way professionals perceive the problem. This has led to (1) different standards for acceptable risks; (2) inconsistent attempt to set up a regional-scale flood management plan beyond the jurisdictional boundaries; (3) absence of a regional-scale agency with licence to share and update information; and (d) lack of forums for dialogue with insurance companies to ensure an integrated approach to flood management.     

ARCGIS‐SWAT: A GEODATA MODEL AND GIS INTERFACE FOR SWAT1

This paper presents ArcGIS‐SWAT, a geodata model and geographic information system (GIS) interface for the Soil and Water Assessment Tool (SWAT). The ArcGIS‐SWAT data model is a system of geodatabases that store SWAT geographic, numeric, and text input data and results in an organized fashion. Thus, it is proposed that a single and comprehensive geodatabase be used as the repository of a SWAT simulation. The ArcGIS‐SWAT interface uses programming objects that conform to the Component Object Model (COM) design standard, which facilitate the use of functionality of other Windows‐based applications within ArcGIS‐SWAT. In particular, the use of MS Excel and MATLAB functionality for data analysis and visualization of results is demonstrated. Likewise, it is proposed to conduct hydrologic model integration through the sharing of information with a not‐model‐specific hub data model where information common to different models can be stored and from which it can be retrieved. As an example, it is demonstrated how the Hydrologic Modeling System (HMS) ‐ a computer application for flood analysis ‐ can use information originally developed by ArcGIS‐SWAT for SWAT. The application of ArcGIS‐SWAT to the Seco Creek watershed in Texas is presented.     

A PROBABILISTIC BENEFIT-COST ANALYSIS OF A ZONING PROGRAM FOR FLOOD PLAINS1

ABSTRACT: Various techniques, one of which is zoning, are used to control the extent of flood damage. The benefit-cost analysis of zoning programs must take into account the random nature of flooding. This paper outlines a method for determining not only the expected value of the benefit-cost ratio, but also the probability of such a zoning program being profitable. It also presents an application of the method to the assessment of the Outaouais Regional Community zoning program.     

MULTICRITERION DECISION MAKING FOR FLOOD CONTROL OPERATIONS: THEORY AND APPLICATIONS1

ABSTRACT: Flood control operations are generally complex in nature, as they are often associated with a large quantity of uncertain factors in combination of noncommensurable criteria. It has been found that the use of fuzzy set theory coupled with multicriterion decision making (MCDM) methods is very promising. In this paper, a multicriterion fuzzy recognition model is established for flood control operations. A subjective preference and iterative weights method is proposed for weight assessment. First, an initial solution of criteria weights, which is directly analyzed from the selected alternatives, is obtained by using the proposed fuzzy recognition model. Then, according to their knowledge related to real time flood operations, operators may modify the initial weights if necessary. When the relative membership grades of alternatives belonging to all rankings are fixed, a defuzzification equation is used to calculate the rank feature value vector. Based on the rank feature value vector, one can evaluate the alternative set. Two flood operation cases are provided to illustrate the application of the proposed model. With the incorporation of the operator's knowledge related to flood operations, the proposed model is flexible in giving weights and realizes more practical flood control operations.     

VENTURA RIVER FLOOD OF FEBRUARY 1992: A LESSON IGNORED?1

ABSTRACT: On February 12, 1992, a portion of the Ventura River, California, flowed through the Ventura Beach RV Resort which had recently been constructed across a major historically active distributary of the Ventura River delta. State and local land-use planners recognized the flood hazards associated with the site, but decision-makers relied on analytical methods developed by the U.S. Army Corps of Engineers and flood-hazard categories developed by the Federal Emergency Management Agency, which did not adequately reflect the mobile nature of the Ventura River channel and distributaries. A better understanding of the historical behavior of the Ventura River could have averted the flood damages experienced in 1992. Low intensity recreational, agricultural, or habitat restoration use of the site would minimize potential flood damages and obviate the need for structural flood protection that would impact the river's natural resources. Continued operation of the recreational vehicle park could result in additional flood damages in the relatively near future; recognizing the limitations of the flood-modeling methodologies used for the Ventura Beach RV Resort could prevent similar miscalculations of flood potential in comparable situations.     

THE COMPLETE DRAINAGE PROGRAM - MORE THAN AN ORDINANCE1

ABSTRACT. An ever increasing number of communities throughout the nation are being forced by Federal legislation and local pressures to adopt land use and control measures related to drainage and flood protection. In many instances communities are hastily adopting ordinances and regulations which later prove difficult to administer and enforce. In addition, many of these ordinances and regulations are not producing the results originally anticipated. This paper discusses the basic elements of a drainage ordinance and evaluates the role of the ordinance in a comprehensive drainage program. This evaluation is based on the results of a two year study of drainage programs in five urban areas which produced a model urban drainage ordinance and recommendations on the administration of the ordinance. Personal interviews and local documents are used as the data base in formulating conclusions and recommendations.     

ESTIMATING PROBABLE MAXIMUM FLOOD PROBABILITIES1

ABSTRACT: Methods of computing probabilities of extreme events that affect the design of major engineering structures have been developed for most failure causes, but not for design floods such as the probable maximum flood (PMF). Probabilities for PMF estimates would be useful for economic studies and risk assessments. Reasons for the reluctance of some hydrologists to assign a probability to a PMF are discussed, and alternative methods of assigning a probability are reviewed. Currently, the extrapolation of a frequency curve appears to be the most practical alternative. Using 46 stations in the Mid-Atlantic region, the log-gamma, log-normal, and log-Gumbel distributions were used to estimate PMF probabilities. A 600,000-year return period appears to be a reasonable probability to use for PMFs in the Mid-Atlantic region. The coefficient of skew accounts for much of the variation in computed probabilities.     

A NONTRADITIONAL METHODOLOGY FOR FLOOD STAGE-DAMAGE CALCULATIONS1

ABSTRACT: This paper presents a new methodology to calculate economic losses from hypothetical, extreme flood events, such as the Probable Maximum Flood. The methodology uses economic data compiled from already-available secondary sources, such as U.S. Census data on magnetic tapes, utilizing microcomputer and other electronic media. Estimates of land elevations are obtained from topographic maps, and flood elevations axe estimated using, for example, a dam breach and flood routing (DAMBRK) model (Fread, 1984). The calculations are performed at a disaggregate spatial scale, by various land use and industrial classification categories. The basic areal units are city blocks (for urbanized areas), enumeration districts, and Census tracts. Depth-damage functions, which provide an estimate of damages as a proportion of the existing value of the structure, are estimated statistically. Computer software (called DAMAGE) is used to combine the economic, flood elevation, and depth-damage information to compute economic losses for different possible flood stages and for different inflow events. Two case studies are presented as illustrations of the method.